The present invention generally relates to magneto-optical recording of informations on a magneto-optical disk and more particularly to a recording system for recording and/or erasing an information signal on and from a magneto-optical disk by a finely focused optical beam while applying a predetermined magnetic field across the disk.
Generally, a magneto-optical recording and reproducing apparatus records an information on a magneto-optical disk by a recording head which irradiates a finely focused optical beam on a recording surface of the disk to cause a local heating of a magnetic recording medium on the recording surface of the disk while applying a predetermined magnetic field across the disk. When erasing the information already recorded on the disk, on the other hand, the direction of the magnetic field thus established has to be reversed. The recording head having a capability of such an inversion of the magnetic field will be referred to hereinafter a magneto-optical recording head or simply a recording head. As a means for producing such a reversible magnetic field, a permanent magnet or an electromagnet is used commonly, wherein permanent magnet is preferred from the view point of reducing the size of the recording head. When a permanent magnet is used for this purpose, a magnet turn-over mechanism for turning the magnet such that the S-pole and the N-pole of the magnet are selectively faced to the recording surface of the disk is needed for changing the direction of the magnetic field.
In order to record or erase the information on or from the magneto-optical disk with reliability, it is necessary to control the foregoing mechanism for turning the magnet such that the N-pole or S-pole of the permanent magnet is precisely directed to the disk. In other words, the permanent magnet has to be directed such that the magnetic flux produced by the magnet is passed substantially perpendicularly to the surface of the disk at a location of the disk where the recording medium is heated by the irradiation of the focused optical beam. For this purpose, the recording head of the conventional magneto-optical disk apparatus has a means for detecting the rotation or turning of the permanent magnet and further for properly controlling the turning of the magnet caused by the foregoing mechanism. In a typical example, a Hall element is used for the detection of the turning of the magnet and for stopping the energization of a motor used in the mechanism for causing the turning-over of the magnet. In another typical example, a limit switch is provided for detection of the turning of the magnet and for deenergizing the motor.
In the first type of the magnet turn-over mechanism in which the Hall element is used for detecting the rotation of the permanent magnet, although there is an advantage in that it is capable of discriminating the N-pole and the S-pole, there is a problem that the accurate detection of the rotational angle or the state of the permanent magnet is difficult. Further, there is another problem that separate Hall elements have to be used for the detection of the magnetic field and for the positioning of the magnet in order to discriminate the magnetic pole and the rotational angle of the magnet at the same time. Such a use of two separate Hall elements complicates the construction of control circuit used for control the magnet turn-over mechanism. In the second type of the magnet turn-over mechanism in which mechanical switches such as the limit switch is used for detection of the rotational angle of the magnet, there is a problem of durability such that the mechanical contacts tends to be worn out when used for a prolonged time period.